I stumbled across an article on a huge complex of man-made tunnels under parts of China that were apparently, like the Great Wall, built to help defend the Northern part of China from invaders.  From the article (click here to read the entire article):

Experts have dug out similar war passages in Yongqing, Xiong county, and Bazhou. The ancient war passages are about 65 kilometers from east to west, 25 kilometers from north to south, which extend through 1,600 square kilometers. When the border between the Song Dynasty and the Liao Dynasty went as far west as Rongcheng county and Xushui county, it is thought that many ancient war passages existed in that area. How far the ancient war passages extended eastwards from Yongqing is still unknown.

These passages were apparently built around 1000 AD. Given the size of this tunnel complex and their apparent usage, perhaps megadungeons aren’t as an unrealistic idea as even those of us who enjoy them have assumed they were.